1. Field of the Invention
The present invention is directed to an amorphous magnetostrictive alloy for use in a marker employed in a magnetomechanical electronic article surveillance system, and in particular to such an amorphous magnetostrictive alloy having a low cobalt content, or being free of cobalt. The present invention is also directed to a method for annealing such a magnetostrictive alloy to produce a resonator and to a method for making a marker embodying such a resonator, and to a magnetomechanical electronic article surveillance system employing such a marker.
2. Description of the Prior Art
Various types of electronic article surveillance systems are known having the common feature of employing a marker or tag which is affixed to an article to be protected against theft, such as merchandise in a store. When a legitimate purchase of the article is made, the marker can either be removed from the article, or converted from an activated state to a deactivated state. Such systems employ a detection arrangement, commonly placed at all exits of a store, and if an activated marker passes through the detection system, this is detected by the detection system and an alarm is triggered.
One type of electronic article surveillance system is known as a harmonic system. In such a system, the marker is composed of ferromagnetic material, and the detector system produces an electromagnetic field at a predetermined frequency. When the magnetic marker passes through the electromagnetic field, it disturbs the field and causes harmonics of the predetermined frequency to be produced. The detection system is tuned to detect certain harmonic frequencies. If such harmonic frequencies are detected, an alarm is triggered. The harmonic frequencies which are generated are dependent on the magnetic behavior of the magnetic material of the marker, specifically on the extent to which the B-H loop of the magnetic material deviates from a linear B-H loop. In general, as the non-linearity of the B-H loop of the magnetic material increases, more harmonics are generated. A system of this type is disclosed, for example, in U.S. Pat. No. 4,484,184.
Such harmonic systems, however, have two basic problems associated therewith. The disturbances in the electromagnetic field produced by the marker are relatively short-range, and therefore can only be detected within relatively close proximity to the marker itself. If such a harmonic system is used in a commercial establishment, therefore, this means that the passageway defined by the electromagnetic transmitter on one side and the electromagnetic receiver on the other side, through which customers must pass, is limited to a maximum of about 3 feet. A further problem associated with such harmonic systems is the difficulty of distinguishing harmonics produced by the ferromagnetic material of the marker from those produced by other ferromagnetic objects such as keys, coins, belt buckles, etc.
Consequently, another type of electronic article surveillance system has been developed, known as a magnetomechanical system. Such a system is described, for example, in U.S. Pat. No. 4,510,489. In this type of system, the marker is composed of an element of magnetostrictive material, known as a resonator, disposed adjacent a strip of magnetizable material, known as a biasing element. Typically (but not necessarily) the resonator is composed of amorphous ferromagnetic material and the biasing element is composed of crystalline ferromagnetic material. The marker is activated by magnetizing the bias element and is deactivated by demagnetizing the bias element.
In such a magnetomechanical system, the detector arrangement includes a transmitter which transmits pulses in the form of RF bursts at a frequency in the low radio-frequency range, such as 58 kHz. The pulses (bursts) are emitted (transmitted) at a repetition rate of, for example 60 Hz, with a pause between successive pulses. The detector arrangement includes a receiver which is synchronized (gated) with the transmitter so that it is activated only during the pauses between the pulses emitted by the transmitter. The receiver "expects" to detect nothing in these pauses between the pulses. If an activated marker is present between the transmitter and the receiver, however, the resonator therein is excited by the transmitted pulses, and will be caused to mechanically oscillate at the transmitter frequency, i.e., at 58 kHz in the above example. The resonator emits a signal which "rings" at the resonator frequency, with an exponential decay time ("ring-down time"). The signal emitted by the activated marker, if it is present between the transmitter and the receiver, is detected by the receiver in the pauses between the transmitted pulses and the receiver accordingly triggers an alarm. To minimize false alarms, the detector usually must detect a signal in at least two, and preferably four, successive pauses.
Since both harmonic and magnetomechanical systems are present in the commercial environment, a problem exists known as "pollution," which is the problem of a marker designed to operate in one type of system producing a false alarm in the other type of system. This most commonly occurs by a conventional marker intended for use in a magnetomechanical system triggering a false alarm in a harmonic system. This arises because, as noted above, the marker in a harmonic system produces the detectable harmonics by virtue of having a non-linear B-H loop. A marker with a linear B-H loop would be "invisible" to a harmonic surveillance system. A non-linear B-H loop, however, is the "normal" type of B-H loop exhibited by magnetic material; special measures have to be taken in order to produce material which has a linear B-H loop. Amorphous magnetostrictive material is disclosed in U.S. Pat. No. 5,628,840 which is stated therein to exhibit such a linear B-H loop. This material, however, still exhibits the problem of having a relatively long ring-down time, which makes it difficult to distinguish the signal therefrom from spurious RF sources.
A further desirable feature of a resonator for use in a marker in a magnetomechanical surveillance system is that the resonant frequency of the resonator have a low dependency on the pre-magnetization field strength produced by the bias element. The bias element is used to activate and deactivate the marker, and thus is easily magnetizable and demagnetizable. When the bias element is magnetized in order to activate the marker, the precise field strength of the magnetic field produced by the bias element cannot be guaranteed. Therefore, it is desirable that, at least within a designated field strength range, the resonant frequency of the resonator not change significantly for different magnetization field strengths. This means df.sub.r /dH.sub.b should be small, wherein f.sub.r is the resonant frequency, and H.sub.b is the strength of the magnetization field produced by the bias element.
Upon deactivation of the marker, however, it is desirable that a very large change in the resonant frequency occur upon removal of the magnetization field. This ensures that a deactivated marker, if left attached to an article, will resonate, if at all, at a resonant frequency far removed from the resonant frequency that the detector arrangement is designed to detect.
Lastly, the material used to make the resonator must have mechanical properties which allow the resonator material to be processed in bulk, usually involving a thermal treatment (annealing) in order to set the magnetic properties. Since amorphous metal is usually cast as a continuous ribbon, this means that the ribbon must exhibit sufficient ductility so as to be processable in a continuous annealing chamber, which means that the ribbon must be unrolled from a supply reel, passed through the annealing chamber, and possibly rewound after annealing. Moreover, the annealed ribbon is usually cut into small strips for incorporation of the strips into markers, which means that the material must not be overly brittle and its magnetic properties, once set by the annealing process, must not be altered or degraded by cutting the material.
A large number of alloy compositions are known in the amorphous metal field in general, and a large number of amorphous alloy compositions have also been proposed for use in electronic article surveillance systems of both of the above types.
PCT Applications WO 96/32731 and WO 96/32518, corresponding to U.S. Pat. No. 5,469,489, disclose a glassy metal alloy consisting essentially of the formula Co.sub.a Fe.sub.b Ni.sub.c M.sub.d B.sub.e Si.sub.f C.sub.g, wherein M is selected from molybdenum and chromium and a, b, c, d, e, f and g are at %, a ranges from about 40 to about 43, b ranges from about 35 to about 42, c ranges from 0 to about 5, d ranges from 0 to about 3, e ranges from about 10 to about 25, f ranges from 0 to about 15 and g ranges from 0 to about 2. The alloy can be cast by rapid solidification into ribbon, annealed to enhance the magnetic properties thereof, and formed into a marker that is especially suited for use in magnetomechanically actuated article surveillance systems. The marker is characterized by relatively linear magnetization response in a frequency regime wherein harmonic marker systems operate magnetically. Voltage amplitudes detected for the marker are high, and interference between surveillance systems based on mechanical resonance and harmonic re-radiance is precluded.
U.S. Pat. No. 5,469,140 discloses a ribbon-shaped strip of an amorphous magnetic alloy which is heat treated, while applying a transverse saturating magnetic field. The treated strip is used in a marker for a pulsed-interrogation electronic article surveillance system. A preferred material for the strip is formed of iron, cobalt, silicon and boron with the proportion of cobalt exceeding 30 at %.
U.S. Pat. No. 5,252,144 proposes that various magnetostrictive alloys be annealed to improve the ring-down characteristics thereof. This patent, however, does not disclose applying a magnetic field during heating.
Many alloy compositions which achieve the above characteristics in their most preferred form and combination (i.e., with all of the above characteristics being optimized) contain relatively large amounts of cobalt. Among the raw materials commonly employed in alloy compositions for producing amorphous material, cobalt is the most expensive. Therefore, amorphous metal products made from an alloy composition with a relatively high cobalt content are correspondingly expensive. In the electronic article surveillance system field, particularly in the field of magnetomechanical surveillance systems, there exists a need for an amorphous alloy which can serve to form the resonator in the article marker which has a relatively low cobalt content, or is cobalt-free, and which is therefore correspondingly reduced in price. The low cobalt content, or the absence of cobalt, however, should not significantly deteriorate the aforementioned magnetic and mechanical properties of the alloy.
Amorphous alloy is commonly cast in "raw" form as a ribbon, and is subsequently subjected to customized processing in order to give the raw ribbon a particular set of desired magnetic properties. Typically, such processing includes annealing the ribbon in a chamber while simultaneously subjecting the ribbon during the annealing to a magnetic field. Most commonly, the magnetic field is oriented transversely relative to the ribbon, i.e., in a direction perpendicular to the longitudinal axis (longest extent) of the ribbon, and in the plane of the ribbon. It is also known, however, to anneal amorphous metal alloy while subjecting the alloy to a magnetic field oriented perpendicularly to the plane of the ribbon or strip, i.e., a magnetic field having a direction parallel to the planar surface normal of the ribbon or strip. Annealing in this manner is disclosed in U.S. Pat. No. 4,268,325. Although a number of cobalt-free alloys are disclosed therein, a number of cobalt-containing alloys are also described. Among the specific examples of cobalt-containing alloy compositions which are provided in U.S. Pat. No. 4,268,325, the lowest cobalt content is 15 at %, and other examples are given wherein the cobalt content is as high as 74 at %. Moreover, the generalized formula which is disclosed in this patent is a cobalt-containing alloy, and is stated to contain cobalt in a range from about 40 to 80 at %. Only some details of the magnetic properties of alloys formed according to this patent are described therein, however, exemplary B-H loops for such alloys are shown. Based on these B-H loops, which are non-linear, the alloys disclosed in this patent would be suitable for use only in harmonic article surveillance systems. Even if some of those alloys had undisclosed magnetostrictive properties, they would still exhibit the aforementioned non-linear B-H loop, and thus would not solve the aforementioned problem of pollution.